We propose to study the origins of DNA replication of ColE1 and R6K plasmid, and the terminus of DNA replication of the latter plasmid by isolating the sites and analyzing their nucleotide sequence. Such study should provide insight into the nature of the initiation and termination steps of DNA replication. In order to define the biological role of the DNA sequences surrounding the initiation sites, we propose to isolate Cis dominant site mutations and deletions by making use of a hybrid plasmid with two origins of replication. The deletions and site mutations will be localized by either fingerprints of RNA transcripts of the appropriate fragments or by analyzing their banding pattern after depurination and depyrimidination in Gilbert-Maxam gels. The results, we hope, will provide insights into the nature of initiation of replication.